Cnidaria: Hydrozoa

The Journal of Venomous Animals and Toxins including Tropical Diseases ISSN 1678-9199 | 2012 | volume 18 | issue 1 | pages 109-115 Structure of nematocysts isolated from the fire coralsMillepora alcicornis ER P and Millepora complanata (Cnidaria: Hydrozoa) A P García-Arredondo A (1, 2), Rojas A (1), Iglesias-Prieto R (3), Zepeda-Rodriguez A (4), Palma-Tirado L (5) RIGINAL (1) Laboratory of Chemical and Pharmacological Natural Products Research, School of Chemistry, Autonomous University O of Querétaro, Querétaro, Mexico; (2) Postgraduate Program in Marine Sciences and Limnology, National Autonomous University of Mexico, Mexico City, Mexico; (3) Puerto Morelos Academic Unit, Institute of Marine Sciences and Limnology, National Autonomous University of Mexico, Cancun, Mexico; (4) Department of Cell and Tissue Physiology, School of Medicine, National Autonomous University of Mexico, Mexico City, Mexico; (5) Microscopy Unit, Institute of Neurobiology, National Autonomous University of Mexico, Querétaro, Mexico. Abstract: Structural characteristics of discharged and undischarged nematocysts from the hydrozoans Millepora alcicornis and Millepora complanata, two fire corals collected in the Mexican Caribbean, were examined using transmission electron, scanning and light microscopy. In this study, we report for the first time images of the nematocysts found in these Mexican Caribbean venomous species. Two types of nematocysts were observed in both species, the more abundant identified as macrobasic mastigophore and the other a stenotele type. Macrobasic mastigophores were present in medium and large size classes while stenoteles appeared in only one size. Key words: hydrocoral, nematocysts, microscopy images, macrobasic mastigophore, stenotele. INTRODUCTION long spiny tubule inside (11). The nematocyst capsule wall resists an extreme intracapsular Hydrozoan cnidarians of the genus Millepora pressure of 150 bar, and upon receipt of an are commonly denominated fire corals since appropriate mechanical and/or chemical stimulus, contact with them immediately causes burning the internal pressure is released (12, 13). This pain, erythema and pustule formation on human causes the eversion of the spiny tubule through the skin (1-3). These hydrocorals are able to induce operculum and injects the venom into the prey. their damaging effects due to the presence The mechanism of discharge of the nematocyst is of nematocysts, the characteristic stinging a very rapid and explosive event that occurs over a organelles used by all cnidarians for defense and period of less than 3 milliseconds. For this reason, capturing prey (4). Several studies have shown this mechanism is considered one of the fastest that the venom contained in the nematocysts processes in biology (14). Moreover, on account from Millepora species display lethal, hemolytic, of their intricate structure, nematocysts were dermonecrotic, and antigenic properties (5-10). considered by Mariscal (15) to be one of the most Each nematocyst is contained in a cnidocyte complex intracellular secretion products known. (nematocyte), the specialized cnidarian cell The most widely used classification of the named after the phylum Cnidaria. The nematocyst variable morphology presented by nematocysts is structure consists of a globular or egg-shape derived from the work of Weill (16, 17). To date, capsule, made up mainly of mini-collagens and the more than 30 particular types of nematocysts glycoprotein NOWA, which has an apical aperture have been identified based on Weill’s work, closed by a cover (operculum) and an inverted which consisted of observations under a light Garcia-Arredondo A, et al. Millepora spp. nematocyst structure microscope, primarily of the discharged tubule 10 m, in the area known as “La Bocana Chica”, and its spine pattern (15, 18). Nematocyst located within the National Reef Park (Parque morphology is potentially of taxonomic Nacional Arrecife) of Puerto Morelos (Quintana importance. For example, of the 25 types Roo, México), in November 2008. Fragments recognized by Mariscal (15), 17 occur exclusively were frozen immediately after collection and in Hydrozoa, and two occur exclusively in stored at –70°C. Anthozoa. The morphology and dimensions of Nematocysts were released from the calcareous the nematocysts are now considered essential exoskeleton as described by Radwan (9) as follows: to any taxonomic description or redescription hydrocoral fragments were stirred in acidic saline of a cnidarian species (18). Furthermore, the solution (0.02 M HCl in 0.15 M NaCl; pH 7) for 24 appearance of the inverted tubule, coiled inside hours at 4°C. Afterward, exoskeletons fragments the capsule, also constitutes a distinct diagnostic were removed and the resultant suspension was characteristic (19). At present, very little is known filtered using a metal screen (0.5 mm pore size). about the nematocysts found in the Caribbean The filtrate was centrifuged for two minutes Millepora species. Therefore, by employing at 200 rpm (5 g), and the supernatant obtained different microscopy techniques, the present was removed by pipette and discarded. The study was undertaken to identify, examine and pellet, containing a mixture of nematocysts and compare the structure of the nematocysts isolated zooxanthellae, was placed in freshly filtered sea from two hydrocorals collected in the Mexican water and re-centrifuged for two minutes at 200 Caribbean: Millepora complanata, a plate-like fire rpm. The supernatant obtained was removed coral (Figure 1 – A); and Millepora alcicornis, a and discarded. This process was repeated one branching fire coral (Figure 1 – B). more time and the resulting pellet was used for microscopic examination. MATERIALS AND METHODS Microscopic Examination Specimen Collection and Isolation of Pellets containing nematocysts were first Nematocists observed directly through light microscopy (LM). For more detailed observations, pellets were Fragments of M. complanata and M. alcicornis studied by scanning and transmission electron were collected by scuba diving, to depths of 4 to microscopy (SEM and TEM). For TEM and SEM Figure 1. Underwater images of the two Mexican Caribbean fire corals: (A) Millepora complanata and (B) Millepora alcicornis. J Venom Anim Toxins incl Trop Dis | 2012 | volume 18 | issue 1 110 Garcia-Arredondo A, et al. Millepora spp. nematocyst structure examinations, pellets containing nematocysts other extreme has a diameter of approximately 2 were fixed in filtered sea water containing 3% µm (Figure 1 – B). glutaraldehyde and 0.1 M sodium cacodylate, The structure of the discharged form of postfixed in 2% OsO4 in cacodylate buffer, and the stenoteles presents an everted broad shaft dehydrated in an ethanol series. with a diameter of about 5.5 µm, and is slightly For TEM examinations, pellets were wider at its base. The basal part of the shaft is embedded in Epon (epoxic resine), the blocks unarmed, but at the distal portion there are three obtained were sectioned into thin slices (60 nm) large spines (called stylets) followed by a short in an ultramicrotome (Mtx RMC®, Boeckler contracted section armed with three helically Instruments, USA) and contrasted with uranyl coiled bands of spines and a thin tubule posterior acetate and lead citrate. These sections were to the shaft (Figure 2 – C to E). The LM image observed in a transmission electron microscope of the undischarged form of this nematocyst type (JEM 1010®, Jeol, USA) operated at 80 kV. For showed the inverted shaft as a straight shape, SEM examination, the nematocysts were dried in folded back within the capsule, with the styletes a critical-point-dryer apparatus (Polaron E5000®, and the small spines pointing away from the Quorum Technologies, UK), covered with carbon operculum (Figure 2 – A). Figure 2 – F is a TEM in an evaporator (JEE4X®, Jeol, USA) and with a image of an undischarged stenotele at the level of thin sheet of gold in an ion sputterer (Polaron the stylets, close to the operculum, showing the 11-HD®, Quorum Technologies, UK), and finally inverted shaft in the center of the capsule. observed with a scanning electron microscope The most abundant nematocyst type found (DSM 950®, Zeiss International) at an accelerating in both Millepora species was the macrobasic voltage of 20 to 25 kV. mastigophore. The structure of the undischarged form consists of an egg-shaped capsule with RESULTS AND DISCUSSION an inverted tubule coiled with an amorphous arrangement, which can be observed in the LM In this study, two types of nematocysts were photographs (Figure 3 – A). The operculum, observed in both Caribbean Millepora species. located in the apical part of the capsule, has a These nematocysts were identified as stenoteles diameter of approximately 2.5 µm in both large and macrobasic mastigosphores according to and medium size classes (Figure 3 – B). In the Weill’s classification (16, 17). Measurements of discharged form of this nematocyst type one the capsule size of the nematocysts, made from observes that the everted tubule is armed with the SEM photographs, showed that in both species three helically coiled bands of spines extended the macrobasic mastigophores were present in throughout the length of the tubule (Figure medium (10.6 – 13.0 x 18.1 – 21.6 µm) and large 3 – C to F). Figure 3 – E shows a macrobasic (17.5 – 21.8 x 25.0 – 33.1 µm) size classes, while mastigophore with the everted tubule completely stenoteles were present in only one size (10.5 – extended; this tubule has a diameter of 1.6

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